This invention relates generally to electrical fuses having a fuse wire spirally wound upon an insulating support core and that is connected between and suspended from the end cap terminals of a cylindrical insulating housing. Particularly, the invention relates to providing improved arc quenching and time delay for such fuses, although the invention is not limited to providing improved time delay and can be applied with advantage to fast acting fuses.
Spiral wound fuses are well known. U.S. Pat. No. 4,445,106 to Shah, assigned to the assignee of this application, discloses an advantageous core material used for such fuses, namely a core made from a ceramic fiber material sold by the 3M Company of St. Paul, Minn. under the mark NEXTEL. This material is specially treated to remove all sizing or binding material that could become conductive under fuse blowing conditions.
In higher current rated, such as 5-30 amperes, fuses, a high energy electrical arc can develop when the fuse wire blows. Such higher current rated fuses will generally use larger diameter fuse wire. The energy of such an arc is directly related to the diameter of the fuse wire so that larger diameter wire results in proportionately higher energy arcs. This electrical arc is sustained by continued overcurrent flow through the fuse and can burn the fuse wire away from the point of blowing to the end caps, at which time the fuse can explode with the attendant hazards to persons in the vicinity of the fuse.
Where the fuse wire is wound with a large or wide pitch (distance between the windings) and there is substantial space between the fuse wire turns along the core, the arc travels around the core. Where the pitch is narrow and the turns are closely spaced, the arc can jump from one turn to the next along the core. This arcing has been a problem because of the expense of applying conventional arc quenching materials, such as filler materials, to spiral wound fuses. As a result, lower cost spiral wound fuses have been limited in the current levels that they can safely protect. Inexpensive means for providing effective arc quenching would thus allow spiral wound fuses to protect against greater overload currents.
If improved arc quenching can be obtained, then the pitch of the spiral winding can be decreased to place more turns per inch on the core without arcing across the turns. This in turn will provide for a longer, heavier gauge fuse wire to be wound on a core with a narrower than usual pitch and provide more heat absorbing mass to increase the time delay available from such a spiral wound fuse. Previously, arcing was avoided by using a wider pitch that required a shorter thinner gauge fuse wire for a given current rating.
Additionally, it could be advantageous if spiral wound fuses, which previously could only be used for slow blow applications, could also be used for fast blow applications.